Abstract

The Li₃Fe₂(AsO₄)_3 − x(PO₄)_x (x = 0, 1, 1.5, 2) solid solution was prepared by the ceramic method. Li₃Fe₂(AsO₄)₃ crystallizes in the monoclinic P112₁/n space group with the cell parameters, a = 8.608(1), b = 12.215(1), c = 8.929(1) Å and γ = 90.76(1)°. The structure consists of a three-dimensional framework of vertex-sharing FeO₆ octahedra and AsO₄ tetrahedra with the lithium ions distributed in three four-coordinated crystal sites. There are two independent iron positions in the structure of this compound, Fe(1) and Fe(2). The cell parameters of the isostructural Li₃Fe₂(AsO₄)_3 − x(PO₄)_x (x = 0, 1, 1.5, 2) phases follow Vegard's law. The IR spectra show the evolution of the intensity for the bands corresponding to the AsO₄ and PO₄ tetrahedra in the solid solution. An isotropic signal with a g value of 2.0 is observed in the ESR spectra of all compounds, in good agreement with the presence of a high spin Fe(III) ion with slightly distorted octahedral symmetry. The magnetization measurements and the magnetic DC and AC susceptibility data show an antiferromagnetic behavior for these compounds, with the presence of a ferromagnetic component below the ordering temperature. T_N decreases from 35.5 to 31 K with decreasing the amount of arsenate. The magnetic results are explained by the existence of a phenomenon of ferrimagnetism in these compounds, due to the existence of two different magnetic sublattices, corresponding to the Fe(1) and Fe(2) sites. The presence of canting between the magnetic moments of these two sublattices has been also detected. This fact agrees with the relative orientation of the FeO₆ magnetic octahedra in the Li₃Fe₂(AsO₄)_3 − x(PO₄)_x (x = 0, 1, 1.5, 2) compounds.